Long-term memory (LTM) is the final stage of the multi-store memory model proposed by Atkinson-Shiffrin, providing the lasting retention of information and skills.
Theoretically, long-term memory capacity could be unlimited, the main constraint on recall being accessibility rather than availability.
Duration might be a few minutes or a lifetime. Suggested encoding modes are semantic (meaning) and visual (pictorial) in the main but can be acoustic also.
Using the computer analogy, the information in your LTM would be like the information you have saved on the hard drive. It isn’t there on your desktop (your short-term memory), but you can pull up this information when you want it, at least most of the time.
Key Takeaways
- Effectively Unlimited: Long-term memory has no known limit on capacity or duration. Most forgetting reflects a retrieval problem, not a lost memory trace.
- Availability vs Accessibility: A memory can be stored yet briefly hard to reach. The right cue often brings it back to mind.
- Two Main Types: Explicit memory (episodic and semantic) can be consciously recalled. Implicit memory, such as procedural skills, priming, and conditioning, shows itself through behaviour instead.
- Meaning-Based Encoding: Long-term memory mainly stores information by meaning rather than sound. Deep, meaningful processing builds stronger memories than rote repetition.
- Evidence From Amnesia: Patients such as HM could not form new declarative memories, yet they still learned new motor skills. This shows declarative and procedural memory rely on separate brain systems.
- Lifelong Retention Is Possible: Bahrick’s classic study found people still recognised former classmates’ names and faces decades later, with some memories holding up after 48 years.
- Memories Can Change on Recall: Retrieving a memory can briefly destabilise it before it is “reconsolidated,” showing long-term memories are dynamic rather than fixed forever.
Types of Long-Term Memory
Long-term memory is not a single store and is divided into two types: explicit (knowing that) and implicit (knowing how).
One of the earliest and most influential distinctions of long-term memory was proposed by Tulving (1972), who separated episodic memory from semantic memory. He added procedural memory as a third system only in 1985, arguing that episodic memory is embedded within semantic memory, which is itself embedded within procedural memory (Tulving, 1985).
Procedural Memory
Procedural memory is a part of the implicit long-term memory responsible for knowing how to do things, i.e., memory of motor skills.
It does not involve conscious (i.e., it’s unconscious-automatic) thought and is not declarative. For example, procedural memory would involve knowledge of how to ride a bicycle.
Semantic Memory
Semantic memory is a part of the explicit long-term memory responsible for storing information about the world. This includes knowledge about the meaning of words, as well as general knowledge.
For example, London is the capital of England. It involves conscious thought and is declarative.
The knowledge that we hold in semantic memory focuses on “knowing that” something is the case (i.e. declarative). For example, we might have a semantic memory for knowing that Paris is the capital of France.
Episodic Memory
Episodic memory is a part of the explicit long-term memory responsible for storing information about events (i.e. episodes) that we have experienced in our lives.
It involves conscious thought and is declarative. An example would be a memory of our 1st day at school.
The knowledge that we hold in episodic memory focuses on “knowing that” something is the case (i.e. declarative). For example, we might have an episodic memory of knowing that we caught the bus to college today.
Priming and Conditioning
Priming and conditioning are two further forms of implicit long-term memory. In priming, earlier exposure to a stimulus makes it easier to process a related stimulus later, with no conscious memory of that earlier exposure (e.g. seeing the word “doctor” speeds up recognising “nurse”).
Classical and operant conditioning create learned associations and emotional responses that shape behaviour automatically. A person can show a conditioned fear response with no conscious memory of the event that produced it.
Cohen and Squire (1980) drew a distinction between declarative knowledge and procedural knowledge.
Procedural and declarative knowledge map onto the implicit and explicit types described above. Procedural knowledge (“knowing how”, e.g. riding a bike or brushing your teeth) runs automatically with little conscious effort. Declarative knowledge (“knowing that”, e.g. that London is the capital of England) requires consciously bringing information to mind.
Evidence for the distinction between declarative and procedural memory has come from research on patients with amnesia. Typically, amnesic patients have great difficulty retaining episodic and semantic information following the onset of amnesia.
Their memory for events and knowledge acquired before the onset of the condition tends to remain intact, but they can’t store new episodic or semantic memories. In other words, it appears that their ability to retain declarative information is impaired.
However, their procedural memory appears to be largely unaffected. They can recall skills they have already learned (e.g. riding a bike) and acquire new skills (e.g. learning to drive).
A landmark example is patient HM (Henry Molaison), who lost the ability to form new episodic and semantic memories after surgery for epilepsy. He still learned new motor skills, such as mirror-drawing, despite having no conscious memory of ever practising them (Scoville & Milner, 1957).
Bahrick et al. (1975)
Bahrick, Bahrick, and Wittinger (1975) investigated what they called very long-term memory (VLTM). Nearly 400 participants aged 17 – 74 were tested.
Participants were asked to list the names they could remember of those in their graduating class in a free recall test.
There were various conditions including: a free recall test, where participants tried to remember names of people in a graduate class; a photo recognition test, consisting of 50 pictures; a name recognition test for ex-school friends.
Results of the study showed that participants who were tested within 15 years of graduation were about 90% accurate in identifying names and faces. After 48 years they were accurate 80% for verbal and 70% visual.
Participants were better at photo recognition than free recall. Free recall was worse. After 15 years it was 60% and after 48 years it was 30% accurate.
They concluded that long-term memory has a potentially unlimited duration.
A strength of this study is that it used meaningful stimuli. Bahrick et al. tested people’s memories from their own lives by using high school yearbooks. The study has higher external validity when compared to studies using meaningless pictures (where recall rates tend to be lower).
But the study did not control for confounding variables (they may have rehearsed their memory of the photos over the years), so any real-world application should be applied with caution.
Critical Evaluation of Long-Term Memory Research
Long-term memory is a well-evidenced concept, but several of its central claims attract legitimate criticism. Four issues are worth noting:
- Untestable claims: “Unlimited” capacity and “permanent” duration cannot be proven directly, only inferred from the fact that no limit has ever been observed.
- A blurred episodic/semantic boundary: Semantic knowledge is often abstracted from episodic experience, so the two may not be fully separate systems.
- Systems versus process debate: Dissociations credited to separate memory systems could instead reflect different types of processing, not separate stores.
- Reconstruction, not reproduction: Episodic memories are rebuilt at recall, shaped by schemas and open to distortion, rather than played back intact.
Untestable Claims About Capacity and Duration
Neither claim can be tested directly. There is no way to prove a store has no upper bound, since that would mean confirming that no future item could ever fail to be stored.
Likewise, “permanent” duration can only be supported by an absence of counter-evidence: no experiment can run for an entire lifetime, so even Bahrick’s finding that some memories survive 48 years shows a long floor, not a proven ceiling.
The deeper problem is that retrieval failure and genuine storage loss look identical from the outside. If someone cannot recall something, that alone cannot distinguish a decayed trace from one that is intact but temporarily inaccessible.
Because every apparent failure can be reinterpreted as a retrieval problem, the claim that nothing is ever truly erased from long-term memory is close to unfalsifiable, and confident statements about what it categorically cannot do should be treated with caution.
Is the Episodic/Semantic Split Genuinely Clean?
The dividing line is not always sharp in practice. Semantic knowledge is often thought to be gradually abstracted out of repeated episodic experience.
A child’s many separate encounters with individual dogs, each an episodic memory in its own right, are eventually distilled into the general concept “dog,” after which the specific encounters themselves tend to be forgotten.
This process means episodic memory can feed into and become semantic memory over time, rather than the two remaining permanently separate stores.
Autobiographical memory makes the blur especially visible: recalling your first day at school mixes a specific episodic scene with semantic facts about schools in general, and it is not obvious which system is doing the remembering.
Because cases like this can be argued either way, some researchers treat episodic and semantic memory as two points on a continuum within one declarative system, rather than as architecturally distinct systems.
Systems Versus Process Views
The taxonomy used throughout this article assumes long-term memory is organised as a set of distinct neural systems, separate stores for episodic, semantic and procedural memory, each supported by characteristic dissociation evidence.
This systems view is associated most closely with theorists such as Tulving and Squire, and it remains the dominant framework in the field.
Process-based accounts offer a rival reading of the same evidence. Rather than crediting a dissociation to two separate stores, they argue it can reflect two different kinds of processing engaged by a task.
For example, conceptual processing, which draws on meaning, versus perceptual processing, which draws on surface features. On this view, a patient who loses one apparent “type” of memory but not another may have lost a kind of processing rather than an entire dedicated store.
The systems-versus-process disagreement remains unresolved for long-term memory, and it parallels a similar store-versus-process debate that runs through the short-term memory literature.
Episodic Memory Is Reconstructive, Not Reproductive
Two classic bodies of work make this point directly. Bartlett (1932), through his War of the Ghosts studies, showed that when people recall a story repeatedly over time, the account is not played back faithfully.
It is actively reconstructed, and unfamiliar or unexpected details tend to be revised toward what the rememberer already expects, guided by pre-existing schemas.
Loftus and Palmer (1974) showed the same reconstructive vulnerability experimentally: simply changing the wording of a question asked after witnessing an event altered what people subsequently reported having seen.
Long-term memory should not be pictured as a video archive: what is retrieved is reconstructed at the moment of recall.
Because recall is reconstructed rather than replayed, this has practical consequences well beyond the laboratory. In eyewitness and forensic settings, misleading post-event information can distort what a witness later reports “remembering.”
This is why interview procedures such as the cognitive interview are designed to avoid introducing contaminating cues, and instead try to reinstate the original conditions under which the event was encoded.
Contemporary Research
Long-term memory is not a fixed archive. Modern neuroscience shows it can be updated, replayed, and even reshaped at the level of individual brain cells.
Memory Reconsolidation
Retrieving a stored memory can return it to a fragile, changeable state. The memory must then be “reconsolidated,” or restabilised, to survive.
Nader, Schafe, and LeDoux (2000) found that blocking protein synthesis during this reconsolidation window weakens a reactivated fear memory in rats. This reframes long-term memory as dynamic rather than fixed, since a stored trace can be altered every time it is recalled.
This discovery has opened new treatment approaches for weakening maladaptive memories in conditions such as PTSD and addiction, by targeting the fragile window that follows retrieval.
Engram Cells
Liu et al. (2012) used optogenetics to reactivate the exact hippocampal neurons active during fear learning in mice. Stimulating these “engram cells” triggered the fear memory directly, without the original cue.
This gave the first direct evidence that a specific memory can be traced to a specific, identifiable group of neurons, rather than being distributed vaguely across the brain.
Related studies have gone further, implanting and reversing false memories in mice by manipulating the same engram cells. Together, this work gives a cellular-level account of both how memories are stored and how they can be distorted.
Sleep and Consolidation
During slow-wave sleep, the hippocampus “replays” newly learned information and gradually transfers it to the neocortex for lasting storage (Squire, Genzel, Wixted, & Morris, 2015).
This gives a biological explanation for why sleep, rather than last-minute cramming, secures long-term learning, and it connects the classic consolidation debate to systems-level neuroscience.
References
Bahrick, H. P., Bahrick, P. O., & Wittinger, R. P. (1975). Fifty years of memory for names and faces: A cross-sectional approach. Journal of Experimental Psychology: General, 104(1), 54–75.
Cohen, N. J., & Squire, L. R. (1980). Preserved learning and retention of pattern-analyzing skill in amnesia: Dissociation of knowing how and knowing that. Science, 210(4466), 207–210. https://doi.org/10.1126/science.7414331
Liu, X., Ramirez, S., Pang, P. T., Puryear, C. B., Govindarajan, A., Deisseroth, K., & Tonegawa, S. (2012). Optogenetic stimulation of a hippocampal engram activates fear memory recall. Nature, 484(7394), 381–385.
Nader, K., Schafe, G. E., & LeDoux, J. E. (2000). Fear memories require protein synthesis in the amygdala for reconsolidation after retrieval. Nature, 406(6797), 722–726.
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